Hi, I've just joined up: I do a lot of offroad bike exploration/ hillwalking /trail running, and have noticed a lot of trails I follow are unmapped on osm. I would upload my gps traces, but they are only useful in sections, e.g parts follow trails but parts are 'cross country' with no path. I'd like to use the traces to edit the map, but I'd be concerned if uploaded that others (I gather from the beginner's guide that 'armchair' editors may use these) then trace parts that are not useful. Is there a way of chopping out or hiding the useless parts, or of using my traces 'offline' to edit the map?

I like to use GPS Prune. You can display several GPXes on it and it displays them on current OSM map. You can then easily see what is and isn't worth using, before trimming and tidying them before uploading. You do need Java installed to run it. Prune is very small and fast app on a windows PC ( i have not tried it on other devices).
https://activityworkshop.net/software/gpsprune/ Prune will also display time and location of each track point which can be useful if you wish to geo-locate some JPEGs as long as you set camera time accurately.

Since you also asked about using your traces "offline" for editing: which editor are you using? You can just drag GPX files into JOSM locally and use them as a background for editing, without uploading the traces.

I do it that way, and very rarely remember to also upload the relevant traces after I'm done editing...

One, the easiest way to trim the data is at recording time. Before starting an offroad section, you could stop the GPX trace recording, or start a new "segment" within the same trace (so when you get to a PC you can review the track at segment granularity, which would be much faster and more accurate). This has all the usual advantages of taking notes while surveying "in the field", such as not coming home, looking at the track, and realizing you don't remember which portions are/aren't offroad.

Two, the traces are useful even in uncropped form: armchair mappers aren't just going to add a highway under any random GPS trace they see, but they will first validate (with e.g. aerial photos) that there is indeed a road in that area. (You can view these aerial photos yourself by opening your favourite map editor.) Also, I assume the offroad traces will be isolated (there will be no other traces parallel to them) whereas the onroad traces will be colocated with many other traces, which will make it easier for mappers to distinguish on- and offroad parts.

You could also add appropriate area tags to the cross-country parts, e.g., landuse=*.